The present invention relates generally to vertebral fixation or defect devices, and more particularly to a dual composition vertebral defect device for insertion into an intervertebral space using minimally invasive techniques, and combining the benefits of support strength and rapid fixation achieved with rigid material, such as titanium, with the advantages of polymeric materials that reduce imaging interference and artifacts.
As shown in prior art FIGS. 6A-10, it is known in the prior art that the spine 100, also known as the spinal column or vertebral column, supports the upper body, allows head, neck, and trunk motion, and includes twenty-four moveable vertebrae 101 including seven cervical vertebrae 102, twelve thoracic vertebrae 104, and five lumbar vertebrae 106, which extend from the skull to the sacrum.
Referring to FIG. 7, with the exception of the first, uppermost cervical vertebra 102, each vertebra 101 has a vertebral body 108, a lamina 110, a spinous process 112, as well as facet structures 114 (which form facet joints), two transverse processes 116, and two pedicles 118, one on each side. Each individual vertebra 101 has a large foramen 120, which forms the spinal canal (not shown) when the vertebrae 101 are in their normal anatomical position forming the spine 100. The spinal cord and major nerve fiber groups pass through and are protected by the spinal canal. A strong fibrous membrane, the dura mater (not shown), also known as the dura, surrounds the spinal cord, nerve fibers, and fluid in the spinal canal.
Referring now to FIGS. 7 and 8, each pair of adjacent vertebrae 101 along with interconnecting soft tissues and an intervertebral disk 128 constitutes a motion segment 122, also known as a functional spinal unit, and the combined motions of many such motion segments 122 constitute overall spinal motion at any one time. The joining of two vertebrae 101 also creates two neuroforaminae 124, also known as intervertebral foraminae, one on each side, each of which is bordered by a facet joint 126 dorsally, a pedicle 118 superiorly, a pedicle 118 inferiorly, and an intervertebral disk 128 ventrally. Each neuroforamina 124 allows passage of large nerve roots (not shown) and associated blood vessels (not shown). The intervertebral disk 128 resides in the space between adjacent vertebral bodies, the intervertebral space 130, also known as the interbody space or disk space. The level of each particular intervertebral space 130 and intervertebral disk 128 is identified by naming the vertebrae 101 superior and inferior to it, for example L 4-5 in the case of the intervertebral space 130 and intervertebral disk 128 between the fourth and fifth lumbar vertebrae 106.
Referring to FIG. 9, each intervertebral disk 128 includes a collection of peripheral concentric rings of strong ligaments known as annular ligaments 132, also known as the annulus, and a softer central area of normally well hydrated material known as the nucleus 134. The annular ligaments 132 are arranged at different angles in alternate layers such that they provide support and stability, resisting excessive vertebral body rotation and axial motion when proper tension is maintained. Although described by some as a cushion, the nucleus 134 is relatively incompressible in a young healthy spine, and thus its major role is to provide support and tension of the annular ligaments 132 to maintain stability while allowing a limited range of motion.
Referring now to FIG. 10, the superior surface and the inferior surface of the lumbar and thoracic vertebral bodies 108 are concave (the shape of the vertebral space 130 shown in phantom). Owing to the shapes of the inferior and superior surfaces of the vertebral bodies 108, the lumbar and thoracic intervertebral spaces 130 and intervertebral disks 128 are biconvex.
Situations arise in which one or more motion segments 122 do not have adequate support or stability, which can lead to pain, deformity, stenosis of spinal canal or neuroforamina, and impairment or loss of nerve function. In some cases, surgical spine fusion is considered. Spine fusion is a process of growing bone between two or more adjacent vertebrae 101 such that the adjacent vertebrae 101 will move only in unison. This process involves placing bone, or material to guide or stimulate bone growth, in proximity to exposed bone of the vertebrae 101, and then allowing time for new bone to grow and form a structurally strong connection, or fusion, between the vertebrae 101. The earliest such procedures took place approximately a century ago, and the procedures have developed over many years, including various attempts to fuse posterior structures of the spine 100 such as the spinous process 112, lamina 110, facet joint 114, and transverse processes 116.
Recently, there has been more interest in fusion involving bone growth directly between adjacent vertebral bodies 101. Large amounts of well vascularized bone are in close proximity, there is a large surface area available, and the inherent compression force applied between vertebral bodies 101 by muscle tension and the upright position of the human body enhances bone formation and strength. The intervertebral disk space 130 has therefore become a major focus in interbody fusion surgery. The disk space 130 is cleared as much as possible, and cartilage and abnormal surface bone, also known as endplate bone, from adjacent vertebral bodies is removed, after which material is placed in the space to promote fusion. However, loose bone fragments do not provide structural support and therefore fusion is often unsuccessful. Structural bone grafts from the patient or donors have been successful, but may give rise to pain and complications if from the patient, and risk of disease transmission if from a donor.
Vertebral defect devices are increasingly used to assist with fusion between vertebral bodies. Such devices are intended to provide support to prevent excessive collapse of space between vertebrae 101 which could result in stenosis of the spinal canal or neuroforamina, progressive deformity, impairment or loss of nerve function, or pain. Such devices also provide at least one compartment to fill with bone, or material which assists in bone growth, in order to maintain close contact with vertebral bone as new bone is encouraged to bridge across the space 130 involved.
Referring to FIG. 7, which shows a single plan view of a vertebra 101, it is known in the art that interbody devices can be inserted from several directions (indicated by arrowed lines) including posterior interlaminar approaches on both sides A, B, transforaminal or partially lateral approaches C, D, anterior approaches E, and straight lateral approaches F.
Efforts have been made to achieve stabilization with vertebral devices having ridges, threads, or grooves on cylinder shaped devices. Anterior approaches (E) allow more access to the disk space, but require more destruction of the annulus. In the lumbar spine 106, bilateral placement of multiple such devices has sometimes been necessary to achieve adequate stability. Cylindrically-shaped devices, inserted through posterior and transforaminal approaches (A, B, C, D) are associated with increased potential for nerve root or dural injury, particularly when drill tubes and reamers are used to prepare the disk space 130 for fusion.
Some cylindrical bone grafts and devices have also been associated with increased subsidence and kyphotic deformities. Subsidence is the sinking of devices or structural bone grafts into adjacent vertebral bodies. Lumbar fusion procedures involving posterior and transforaminal insertion have relied more on impacted devices, which do not provide immediate stabilization except by some degree of distraction, which is naturally lost by any subsidence, and they have been subject to excessive subsidence in some patients. It has been taught in the art that these devices should be inserted bilaterally to offer proper support.
Vertebral devices have been constructed with polymers such as PEEK and carbon fiber/PEEK combinations. Such devices have the advantage of minimal interference with future imaging studies whether by x-ray, CT scan, or MRI scan. Such devices usually have simple implanted metal markers in front and back to allow limited visualization of their position with x-rays or the like. They are made with thick, vertically straight walls to provide support strength, but once they subside a small amount the straight walls offer no effective resistance to excessive subsidence. The surface area provided for fusion is also limited by the thick walls. Polymer material in current use does not allow construction of sharp edges and fixation elements and does not allow for varied shapes which might solve many of the problems with subsidence.
Lumbar surgery is experiencing an evolution to minimally invasive surgery. This trend has led to the need for devices that can be inserted through small portals or working channels, usually through one small incision on one side of the patient's body. In lumbar fusion, vertebral devices should assist with rapid fixation to minimize the need for extensive additional fixation with bone screws, rods, and plates. Surgeons generally prefer posterior approaches for lumbar spine procedures due to the morbidity of anterior approaches, which also causes adhesion of major vessels and makes repeat anterior surgery very dangerous and even life threatening. When posterior lumbar fusion is performed, there is an opportunity to approach the spine 100 through the neuroforamina 124 and insert interbody devices through a small space free of vital structures which is located lateral to the dura in the canal, medial to the large nerve root passing through the neuroforamina 124, and bordered inferiorly or caudally by the pedicle 118 of the vertebra 101 below the involved disk space 130. The distance between peripheral edges of the vertebrae 101 at the disk space 130 is often small so that entry of devices has required considerable bone removal to safely impact devices into the disk space 130. If a device with a distal end of 3 mm or larger in height or transverse dimension is impacted into the disk space 130, it will frequently displace medially or laterally, and involve nerve structures. When working through a small portal or working channel, it is difficult to see this displacement, which makes some devices dangerous in this regard.
Conventional vertebral devices do not lend themselves to be used in minimally invasive surgery because open bilateral surgery is often necessary. When inserting a conventional vertebral device, a distraction tool is used to open the disk space on one side of the spine 100 to allow insertion of the vertebral device on the other side of the spine 100. Alternatively, pedicle screws on one or both sides of the spine 100 may be used with a distraction instrument to spread the disc space open for insertion of one or more vertebral devices.
It is therefore desirable to provide an impacted vertebral defect device designed to achieve rapid fixation while preventing excessive subsidence. The device should reduce the potential for neural injury during insertion, and reduce or eliminate the need for bilateral lumbar pedicle screws. The device should have excellent support strength, but limit the amount of interference with future imaging studies. It is also desirable to provide a device that can be inserted during minimally invasive surgery and placed through an incision and directly into the disk space without manual or mechanical separation of the vertebrae and preferably asymmetrically obliquely across the disk space. It is further desirable for the device to have distinct elements that show its orientation and angulation in the disk space with minimal time and effort using imaging studies, such as with single plane lateral fluoroscopic x-ray.